During the course of measuring the binding enthalpies of the anthracycline drug daunomycin to DNA hosts in the 10 to 20 micromolar range (where the monomeric form predominates), we found that the experimental protocol could be simplified greatly by adding a third inlet line to the existing two inlets in each channel of the microcalorimeter. This change will allow a dual-ratio mixing scheme to be used, in which we can keep the concentration of one reagent (DNA) constant and vary the concentration of the other reagent (i.e., drug) by ratioing the other two lines.